Susceptibility and antibody production against pathogenic and vaccine strains of the haemoflagellate, Cryptobia salmositica were investigated in five full‐sib families (A–E) of Atlantic salmon, Salmo salar . Humoral response and susceptibility of families were compared within three treatments: infection, vaccination and vaccination followed by challenge. Parasitaemias caused by the vaccine strain of C. salmositica were conside...